Traditionally, baby monitor systems have been one-way communication devices. Specifically, a transmitting unit is provided for an infant's room and a receiving unit is provided for an adult so that the adult is able to monitor or hear sounds detected by the transmitting unit. In some of these systems, the transmissions are continuous, while in others a sound-activated mode may be provided in which transmissions are only made when the transmitter unit detects sound waves greater than a predetermined magnitude.
Such monitoring systems have typically been one-way communication devices because there has been little demand to communicate from the adult unit back to the child unit, since the devices are most popular for use with newborn infants. In the case of monitoring newborns, parents typically want to hear every sound generated in the infant's room on a continuous basis without interruption. It has not typically been desired to provide the ability for the parent to speak to the newborn; most likely because the newborn may not be reassured by simply hearing the parent's voice replayed over a monitor, but rather may be frightened by hearing the parent's voice without seeing their face at their bedside.
As infants grow out of the newborn stage, however, it may be desirable for the parent to be able to talk to the infant either to reassure them or to provide instructions to the infant. For example, the parent may want to reassure a crying child and/or tell them to stop crying and go back to sleep. If this functionality were available on a monitoring system, the parent could avoid having to get out of bed in the middle of the night and go to the infant's room. Unfortunately, this functionality requires two-way communication.
Two-way communication devices are typically either full-duplex or half-duplex systems. Full-duplex systems allow for simultaneous transmission and reception by each unit of the system. In other words, the two users of such a system can talk and be heard simultaneously. In order to accomplish this functionality, full-duplex systems typically involve a pair of transceivers (units which are both a transmitter and a receiver) utilizing two separate communication channels, one for each direction. For example, a full-duplex system may be realized on two discrete frequency channels, or on two different sets of electrical conductors or wires connected between the two monitoring units. In the case of radio frequency (RF) communication between the two monitoring units, the two discrete frequency channels must be sufficiently offset from each other to allow full bandwidth communication simultaneously. Such capability in monitoring systems can be relatively expensive to provide. Unfortunately, because of the competitive market for juvenile products, such systems are deemed too expensive to include in baby monitors In addition, with the plethora of consumer, commercial, broadcast, government, and military applications involving RF communications, the RF spectrum is becoming very crowded and two discrete frequency channels may not be available.
Typically, half-duplex systems, which provide for communication in only one direction at a time, include a pair of transceivers that each include a "push-to-talk" button. These devices are commonly known as "walkie-talkies." Such units are designed to function as receivers unless the push-to-talk button is actuated, at which point the transceiver transmits while the button is actuated. There are several issues that arise with such half-duplex systems. Since it is possible for each transceiver to be placed into the transmit mode at the same time, it is possible for one user to attempt to communicate to the other user at the same time that the other user is attempting to communicate to the first user. Clearly, no communication will be made during such instances. Thus, it can be appreciated that these subsystems must be used properly to provide useful communication. To improve the communication between users of half-duplex systems and increase the efficiency thereof, certain hand-off protocols have been developed and continue to be used by serious radio operators. This may include, for example, the use of a certain word to begin a transmission which acknowledges the receipt of the last incoming transmission with a code word such as "Roger." In addition, the user may signify the end of a transmission by using a code word such as "Over" to indicate to the receiving party that the channel is now clear for the receiving party to communicate back to the transmitting party. Unfortunately, such techniques are not likely to be used correctly by many consumers and especially by young children. In addition, young children may not even be able to actuate the talk button.
For these reasons, in order to provide two-way communication employing a half-duplex system without push-to-talk buttons, more sophisticated techniques have been developed. Specifically, a technique known as "sniffing" has been developed for a transceiver to determine when the opposite transceiver is attempting to transmit. This technique is embodied in an earlier product sold by the predecessor of the assignee of the present invention, and known as the "Two-Way Portable Family Intercom," offered as Gerry Model No.605. This system featured several different communication modes, including a one-way mode, a two-way mode, a night mode, and a silent mode. During the two-way mode a child unit acted primarily as a transmitter and a parent unit acted primarily as a receiver. On a periodic basis, however, the child unit would cease transmitting and compare the received signal to a predetermined threshold. If the received signal (primarily, if not entirely, noise) was greater than the predetermined threshold, then the child unit could assume that the parent unit was not transmitting and only noise was being detected. In this case, the child unit would resume transmitting. If, however, the child unit determined that a signal less than the predetermined threshold was being received, then the child unit could assume that the parent unit was transmitting at that time. The child unit would then switch over to receiver mode in which the received signal was converted to an audio signal and played through a loudspeaker in the child unit so that those in the vicinity thereof could hear sound waves representative of the signal transmitted by the parent unit.
There were two disadvantages with this system. First of all, during each of these periodic "sniff" periods when the child unit was not transmitting a signal, the receiver of the parent unit would amplify and play over its loudspeaker a noise signal due to the lack of a transmitted signal from the child unit. This noise signal sounded like a periodic, short click or period of hiss. While it did provide reassurance to the parent that the child unit was in range and was properly conducting its sniff function, the sound was found to be undesirable by some. Furthermore, due to the prevalence of other consumer devices and the like which transmit RF signals, the child unit would occasionally sniff a transmission which did not come from the parent unit. Nevertheless and undesirably, the child unit would transition to the receive mode and either no signal, noise, or an unintended signal would be detected and played over the loudspeaker of the child unit. In addition, during this period when the child unit was in the receive mode, the parent unit would continue to play noise through its loudspeaker.
In the field of communication devices, it is known to provide non-audible or hidden tones in the audio signal as a code for use by the receiver. These hidden tones are typically at a frequency range outside of the core frequency range of human speech and thus outside of the frequency range amplified and played for the user over the loudspeaker of the receiver. For example, radio systems for taxis include hidden tones to turn on only the radio receiver in the taxi requested. Commercially-available circuits are available that incorporate such techniques and may be referred to as Continuous Tone Control Squelch Systems (CTCSS).
It is against this background and the desire to solve the problems of the prior art that the present invention has been developed.